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Sharks change their blood volume depending on the saltiness of the water they're swimming in, according to new research.

So sharks must continuously regulate their blood and other body-fluid volumes as water salinity changes.

The paper, in the latest issue of the journal Comparative Biochemistry and Physiology, provides one of the most extensive looks at this phenomenon, and points out how massive ice melt triggered by warming could spell trouble for sharks.

Although sharks absorb water through their gills, they may also drink seawater and expel shark urine to keep their systems in sync with their environment.

"Body fluid regulation is important for sharks in the same way it is important for humans," says lead author Gary Anderson, an assistant professor of biological sciences at Canada's University of Manitoba.

"Without appropriate regulation of body fluid volumes, physiological systems, such as the cardiovascular and renal systems, would not function optimally and therefore the animal would suffer as a result," he says.

Shark blood consists of cells and plasma, a protein, water and a mineral-containing substance.

Anderson and his team say that sharks in low salt or freshwater environments have much higher blood volumes than sharks in salty water.

When shark plasma amounts rise, their cell counts remains the same, so sharks in fresh water essentially develop watery blood in response to certain surroundings.

Many ways to change volume

The researchers analysed earlier studies and data on bull sharks, bamboo sharks, several dogfish and other shark, ray and skate species.

They conclude that sharks accomplish blood and fluid volume changes through at least five processes.

The first involves water entry through the shark's permeable gills. But sometimes the absorption is not enough to sustain the shark.

When two types of dogfish were transferred from 80% salt water to 100% salt water, they gulped down water like a thirsty human. Drinking became necessary as less water, per volume, was present in their changed environment.

Another way that sharks regulate their blood and fluids is by varying their amount of urine output.

As a shark's blood pressure rises and falls, "the renal system is activated to increase or decrease urine output and therefore excrete or retain water", the scientists write.

In other words, the shark either holds it, or lets loose, in response to water salinity.

Bizarre gland

To assist the kidneys, sharks also have a rectal gland, which is a made of specialised salt-secreting tissue. This too affects shark blood volumes, but its full contribution has yet to be determined.

Finally, the researchers discovered that the intestines play a vital role in keeping shark fluids in balance with their habitat.

Although sharks live in water, they can become internally dehydrated, due to excess salts, so intestinal cells have to work hard to excrete the excess salt while retaining water.

While these complex processes have allowed sharks to adapt to natural environmental changes over the millennia, scientists say global warming could overtax them.

According to Michael Schlesinger, professor of atmospheric sciences at the University of Illinois at Urbana-Champaign, glacial, ice sheet and other ice melts could add fresh water to the North Atlantic, leading to a "smoking gun" for sharks, other marine life and land dwellers.

"So far, the salinity change is small, but we could be standing at the brink of an abrupt and irreversible climate change," Schlesinger says. "What we are seeing is very worrisome."